Classical theory

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Classical theory has two distinct meanings in physics:

1. In the context of quantum mechanics, "classical theory" refers to theories of physics that do not use the quantisation paradigm, particularly Newtonian mechanics (which is also known as classical mechanics). General relativity and special relativity are also considered to be "classical" in this sense.
2. In the context of general and special relativity, "classical theory" refers to classical mechanics, and other theories which obey Galilean relativity. Light and other electromagnetic pheneomena cannot be correctly modeled in such a theory. Traditionally, light was reconciled with classical mechanics by assuming the existence of a "stationary" medium through which light propagated, the luminiferous aether.

The existence of these two distinct meanings of the term can lead to confusion: special relativity is a "classical theory" in the first sense, but its predictions are more accurate than "classical theory" in the second sense.

In other contexts, "classical theory" will have other meanings—if a current accepted theory is considered to be "modern", and its introduction represented a major paradigm shift, then previous theory (or new theories based on the older paradigm) will often be referred to as "classical".